1. Field of the Invention
Aspects of the present invention relate to a recording medium, and more particularly, to a hologram recording medium on which data is recorded using interference fringes of an object beam and a reference beam.
2. Description of the Related Art
Recently, rewritable optical disks including phase change optical disks and conventional optical disks have become widely used as information recording media. In order to increase the recording density of such a rewritable optical disk, the diameter of a beam spot and the distance between adjacent tracks or adjacent bits must be reduced.
Although the recording density of rewritable optical disks has increased, the recording density of an optical disk is physically limited by a diffraction limit of a beam for recording data on a surface. Accordingly, the optical disk using a three dimensional recording in which the depth direction of the disk is used has been developed to increase recording density. Such an optical disk is called a hologram recording medium.
A hologram recording medium has a large capacity due to the three dimensional recording region and can be used at high speed due to a two dimensional recording/reproducing method, and has attracted public attention as a next generation computer recording medium. The holographic recording medium uses photosensitive material, such as a photopolymer or photorefractive crystal, to record interference patterns caused by the object beam and the reference beam. For example, a hologram recording medium may be formed by inserting a recording layer formed of a photopolymer, between two sheets of glass. In order to record data on such a hologram recording medium, an object beam and a reference beam corresponding to data to be recorded are irradiated on to the hologram recording medium to form interference fringes in the photopolymer. The object beam includes data or other information. In order to reproduce data from the hologram recording medium, a reference beam is irradiated onto the interference fringes to extract optical data corresponding to the recorded data.
In addition, cube shaped and card-shaped hologram recording media have been developed. In the case of a card-shaped hologram recording medium, a recording capacity is increased by increasing the number of recording layers on which a waveguide is formed. Japanese Laid-open Patent No. 2000-4434 discloses a method of recording data on a hologram recording medium.
In a conventional method of recording data on a hologram recording medium in multiple angles by using an object beam and a reference beam, the reference beam should be incident on a recording layer of the hologram recording medium at a precise incidence angle when recording/reproducing data. Thus, an optical device, such as a mirror, should be precisely controlled, resulting in an increase in size and cost of the optical device. Accordingly, a method of recording data on a hologram recording medium by focusing an object beam on at least one small area of a recording layer and focusing a reference beam on the small area is required. In this case, the hologram recording medium includes at least one core layer, which is interposed at a portion of the recording layer having a low refractive index to waveguide a beam, an optical waveguide which has at least one diffraction grating layer disposed at an interface between the core layer and the portion of the recording layer having a low refractive index or in the core layer, and at least one recording layer disposed outside of the optical waveguide to transmit a reference beam to the optical waveguide.
However, when a diffraction beam from the core layer is used as a reference beam, the reference beam is irradiated onto an entire surface of the recording layer of the hologram recording medium. Thus, the reference beam is incident on an area of the recording layer to which data is not to be recorded and the area to which data is not to be recorded is exposed to light not used to record data thereon. In spite of the exposition to light, data may be recorded on the recording layer. But, in this case, the quality of the recorded data may deteriorate. In addition, the reference beam may be input to an optical detector, which detects a reproduction beam, and the reference beam acts as noise, thus the data cannot be precisely reproduced. In a conventional method of recording data on a hologram recording medium, a diffraction grating layer and a recording layer cannot be separated, resulting in an increase in manufacturing costs and resulting in a security problem due to the easiness reproduction of data by a third person.